1. Field of the Invention
The invention relates to the field of disk drive systems and, in particular, to a magnetic disk having patterned servo regions that assist in aligning a slider with data on the magnetic disk.
2. Statement of the Problem
Many computing systems use magnetic disk drive systems for mass storage of information. Magnetic disk drive systems typically include one or more sliders that include read and write heads. An actuator/suspension arm holds the slider above a magnetic disk. When the magnetic disk rotates, an air flow generated by the rotation of the magnetic disk causes an air bearing surface (ABS) of the slider to fly at a particular height above the magnetic disk. The height at which the slider flies depends on the shape of the ABS. As the slider flies on the air bearing, a voice coil motor (VCM) moves the actuator/suspension arm to position the read head and the write head over selected tracks of the magnetic disk.
The magnetic disk includes data sectors and servo sectors. The servo sectors include servo data that is used to provide sector information, timing information, positioning information, etc. For example, the servo data may include a sector ID, a track ID, and a servo burst. The sector ID is a field in the servo sector that contains a binary code identifying the sector. Servo sectors are generally labeled sequentially around a track (e.g., from sector #1 to sector #250 if the track has 250 servo sectors). By reading the track ID, the read head can determine what track it is over. A servo burst is a field in the servo sector that contains a specially designed pattern, which, when read by the read head, provides information about the position of the read head relative to the center of a specific track. By reading the servo burst, the controller can determine how far from the center of the track that the read head is. These two pieces of information are used by the controller to determine where the read head, or more specifically the slider, is on the disk. By reading the sector ID, the controller determines which part of the revolution the read head is over. As the magnetic disk makes a revolution, the read head passes over servo bursts and servo sectors. This servo data is fed back to a control system via a read signal that is used to generate a timing signal, a position signal (e.g., a quadrature signal), etc. The timing signal is used to control the VCM and the rotational speed of the magnetic disk. The position signal is used for centering the read head and write head over the center of a track (i.e., to keep the heads aligned with the data). This servo data is provided to a control system via a read signal and is used to generate a timing signal for spindle speed control and a position signal for positioning the head.
Servo patterns include pattern elements that occupy a relatively large lateral spatial extent of the magnetic disk, both down track and cross track, as compared to the size of a data bit on the disk. For example, in a conventional magnetic disk, two magnetizations of polarization are used for recording the servo patterns. The servo headers generally have large areas of uniform magnetization of polarization. Each region may be much larger when compared to a single data bit on the disk.
Patterned magnetic disks have emerged recently to enhance the recording density by providing better track isolation or bit isolation. For example, nano scale nonmagnetic grooves may be patterned in a magnetic disk by removing the magnetic material leaving behind “lands” of magnetic material. By patterning grooves in the magnetic disk, tracks can be more clearly distinguished and thus made narrower to increase the areal density of data on the magnetic disk. Two forms of patterned magnetic disks exist: Discrete Track Media (DTM) and Bit Patterned Media (BPM). In BPM, individual bits may be patterned via cross grooves of nonmagnetic material (e.g., track grooves and crossing bit grooves that leave behind “islands” of magnetic material). In each of these, servo patterns may be generally created as part of the overall disk patterning process. In DTM, discrete tracks are patterned into the magnetic disk. One common approach to creating servo patterns is to pattern the magnetic material of the disk into bit lands such that a Direct Current (DC) magnetization (i.e., unipolar magnetization) of the entire disk may be used to create readable servo patterns via the signal contrast between the presence and absence of magnetic material.
When conventional servo patterns (e.g., servo burst patterns, sector ID patterns, cylinder ID patterns, synchronization and automatic gain control patterns, etc.) are created in this manner, many regions of nonmagnetic material have different shapes and sizes. This creates a significant challenge for planarization of the magnetic disk, which is important as it creates a reliable head-disk interface. The problem is that many of the available planarization methods have difficulty dealing with filling relatively large depressions in the disk that result from implementing the servo patterns. For example, certain planarization methods impose design rules on patterned media. For liquid-based planarization, all non magnetic grooves should be configured at or below a specified width that allows for the liquid to planarize the grooves through capillary forces. For dry planarization, such as vacuum deposit/etchback planarization, the ratio of magnetic land widths to non magnetic groove widths needs to be constant everywhere (“dry planarization design rule #1”). It is also advantageous to ensure that magnetic land and non magnetic groove widths are constant everywhere (“dry planarization design rule #2”). However, conventional servo patterns do not allow for this because of their widely varying shapes and sizes.
Additionally, with patterned media, all of the servo patterns would preferably rely on DC magnetization during fabrication of the disk to provide usable servo signals for the life of the drive. Such would have the effect of not requiring additional servo writing, thereby saving time and money during the disk manufacturing process. Some methods of planarization (e.g., vacuum deposition and liquid polymer fill), however, are sensitive to the density and width of topographic features. For example, if a process is optimized to fill nonmagnetic grooves between patterned tracks in a data recording region of a DTM disk, the same process may produce unsatisfactory results on patterned servo data because the density and widths of the servo patterns can vary substantially from that of the DTM data tracks. A dip-coat/liquid spin-on process is particularly attractive from a cost and simplicity point of view; however, it often fails to fill features of servo patterns. One etch back process that may be used to avoid such limitations is chemical mechanical polishing (CMP); however, CMP is relatively expensive and difficult to implement, thereby adding to the overall cost of the produced magnetic disk. Accordingly, a need exists to create servo patterns to take advantage of the patterning of magnetic disks patterned while remaining compatible with planarization methods.